Central blood pressure has traditionally been measured by the insertion of a hollow needle or catheter into a functioning artery. The needle tip is kept clear of clotting blood by a small amount of an anti-coagulant such as hepperin or another similar compound that inhibits clotting of the blood. A tube is connected from the needle and any entrapped air is vented by way of a valve to the atmosphere. Another port on the valve connects with a traditional pressure sensor device. This sensor outputs a direct current voltage that can be calibrated to pressure.
The disadvantages of such invasive blood pressure monitoring systems include the necessity of an anti-coagulant, which is extremely dangerous in bleeding patients, hemophiliacs, and small infants, and cross contamination. In addition, finding a patentable artery to be catheterized may be difficult or impossible. Moreover, a frequent result of such invasive procedures is the destruction of the artery at the point of entrance of the catheter. In view of these problems with invasive blood pressure monitoring systems physicians and others have sought to develop blood pressure measurement systems that are non-invasive, easily set up, and do not require constant attention to infusion rates and patentability.
Previous devices such as those offered by Dynamap, Siemens, Datascope, and other manufacturers utilize an oscillometric method for obtaining blood pressure at specific intervals. i.e. every 3 to 10 minutes. This method operates on the empirical correlation between actual blood pressure and the point at which heart sounds (Korotkoff sounds) are heard in an artery as pressure is released from an occluded artery. Typically the artery is occluded by pressurizing a cuff containing an internal air bladder that is wrapped about the arm or leg. As the air is released from the cuff the initial Korotkoff sounds heard correspond to the blood turbulence that is produced as the artery begins to open and blood starts to resume its flow through the artery. This is the systolic or high point of the blood pressure. As the artery opens further turbulence diminishes, heart sounds get lower in intensity, and the diastolic or low point is noted. Conventionally, the mean blood pressure is usually about the average of the two or slightly lower i.e., the mean pressure is at the level of 1/3 of the difference between the diastolic and systolic values.
The disadvantages of the oscillometric approach is that relatively infrequent readings of pressure are obtained when compared to the continuous blood pressure reading obtained from the "A" line arterial method. Continuous readings are mandatory in some operations and certainly desirable in most. If an attempt is made to cycle these "occlusive" pump up monitors too frequently, i.e. less than 3 minutes apart, nerve trauma and permanent damage can result to the limb involved.
Another method of monitoring blood pressure is to utilize the occlusive method for obtaining the initial readings and then to monitor arterial pulsations at a much lower cuff pressure i.e. 20 to 40 mm hg. In one such system, a sensor is placed on the wrist and adjusted by means of a screw to optimize position over the radical artery in the wrist. The sensor monitors the pulsatility of that artery based on the calibration of a standard occlusive measurement. Other systems rely on pulsatile information from an artery, but utilize the brachial artery in the lower upper arm near the elbow. Such systems determine the patient blood pressure by the initial occlusive measurements, i.e. diastolic, mean, systolic, and apply or assign these measured pressures to the trough, 1/3 above trough, and peak respectively of the very much lower amplitude pulsations measured from the low pressure cuff sensor. Then various "corrective" factors and scaling algorithms are employed, based on empirical data, to bring the calculations into line with measurements obtained by the invasive A method. As a result, the system continuously monitors pressure variations in the low pressure cuff sensor and continuously calculates an actual pressure based on initial occlusive measurements and its internal algorithms. Applicant believes that the use of pulsatile amplitude measurement as a sole basis for on line continuous computation of the vital sign of blood pressure is dangerous, produces reversed trends, and can lead to the administration of medication that is the opposite of that required for critical and nominal care in controlling blood pressure.
One disadvantage of the use of pulsatile measurement based on initial occlusive determination arises from the fact that the stress/strain curve for the arterial wall is a complex function. If the artery were looked at alone in space, it would still be complex since its walls are comprised of several layers, some of which are smooth muscle that are susceptible to the sympathetic nervous system responses and can react to various stimuli from the body. These vasoconstrictor fibers surrounding the major arterial and venous blood vessels can greatly change the resistance, and compliance or elasticity of the measured artery. Obviously, if the elasticity of the pulsating, measured artery were to change, the amplitude of the pulsations could change also. An example of what happens when this occurs demonstrates the limitations of the pulsatile amplitude method of continuous pressure measurement.
It is known that when peripheral resistance increases the impulses from the medullary vasomotor center to the vasoconstrictor fibers mentioned above increase in rate. This is known to raise arterial pressure and is in fact the primary means that the body utilizes to regulate pressure. As the fibers constrict about their artery, its compliance, elasticity, and pulsatile ability decreases past a certain pressure point (different for all individuals but relatively low in patients that are compromised with hypertension, i.e. (calcified, hard, or clogged vessels)). This effect will cause a lowering of pulsatility for a rise in pressure. This is exactly the opposite of what is supposed to happen according to the pulsatile measurement technique. The pulsatile technique is based on the premise that if the amplitude is increasing, then the pressure is increasing. However, the converse is also true, i.e. that as peripheral resistance goes lower making the arterial wall more compliant, the pulsatility will increase. This erroneously is interpreted by monitors utilizing the pulsatile method as an increase in pressure when, in actuality, the pressure is going down.
Another disadvantage of the pulsatile measurement technique arises because peripheral changes in vessel resistance also produce blood volume changes in the arm or leg. Any change in volume will cause a loss of calibration with respect to the original occlusive reference. For example if more blood enters a fixed volume i.e., the upper arm, the skin acts like a balloon being stretched. As more blood enters the arm, the force of this interarm pressure becomes greater, acting as a constricting force vector on any pulsations of the vessels in the arm aimed back at the center of the vessel. Thus, as interstitial fluid, or blood, increases in the appendage, the ability of the vessels to pulsate diminishes. The pulsatile measurement technique detects this and a pressure decrease which unfortunately is the opposite of what is actually occurring, since pressure in the limb is obviously increasing.
Still further, the non linear relationship of the arterial stress/strain curve can, as the limited working range of the pulsatile method moves up or down on this curve, produce trends opposite that of what is actually taking place. These are caused whenever cardiac output, i.e. volume or peripheral resistance changes occur. These changes occur frequently under anesthesia, intubation, and operating procedures.
Finally the indigenous signal to noise ratio is very low for the desired pulsatile measurements. The problem is that the power spectrum of the typical arterial pulse as acquired by the non-invasive pressure transducers is the same as the muscle and motion artifacts in many cases. Therefore, they are inseparable in either the time or frequency domains. No amount of software or signal processing can restore the data during these often occurring situations. The data is, therefore, discontinuous in many instances, e.g., during an operation or muscle tremor, when it is most needed.
The above described difficulties render the pulsatile measurement technique useful only in those few patients who are not undergoing any physiological change and who have excellent arterial compliance not compromised by vascular hypertension or disease. There are few patients that have these qualities for any length of time useful in realistic monitoring situations.
It is an object of the present invention to provide a more non-invasive method and apparatus for continuously monitoring patient blood pressure.
It is another object of the present invention to provide accurate and continuous blood pressure readings.
Another object of the invention is to provide for continuous blood pressure readings by a method and apparatus which is readily attached to a patient.
In accordance with the present invention blood pressure is measured non-invasively, i.e. without penetration into the body, on a continuous basis. Continuous is defined herein as a moment by moment collection of data as distinguished from intermittent or other devices operating at fixed time intervals. The invention is also designed to overcome the disadvantages of the before-mentioned pulsatile amplitude systems.
The apparatus includes a pressure cuff adapted to be placed about a patient's limb. The cuff is connected to a pressure transducer and is initially used to establish systolic and diastolic pressure values by occlusion of the limb. Pressure in the cuff is then decreased to below venous pressure, e.g. 10 mmhg, while the transducer produces pulsatile (A.C.) and absolute (D.C.) output signals. The central processing unit to which the transducer is connected calibrates the AC signal by assigning the previously measured systolic and diastolic pressure values to the peak and trough of the A.C. signal. The D.C. signal is representative then of the mean arterial pressure as the limb expands and contracts due to the increase or decrease in blood volume associated with increasing and decreasing blood pressures. Variations in the amplitude of the AC component of the transducer signal are used as an indication of the absolute value of the systolic peak and diastolic trough about the mean and are not used to calculate the mean.